A large number of sensing approaches and transducer types have been developed for the determination of the level of liquids in open or closed vessels or containers. Not only is the knowledge of the level itself important, but other measurements can be inferred from the level. For example, if the container geometry and dimensions are additionally known, the volume of the liquid can be determined. If additionally, the density of the liquid is known, its mass can be calculated.
Relatively high technology methods to sense liquid level by measurement of properties such as distance, time, and pressure include: sonic and ultrasonic surface reflection, optical surface reflection, optical transmission, rotation of polarized light, capacitive property sensing, dielectric conduction between electrodes, inductive property sensing, mechanical resonance damping, float device containing a magnet affecting pulse reflection of a ferromagnetic strip, float containing an end of a linear variable differential transformer, pressure sensor at bottom of the tank, differential pressure sensor bubbling tube at bottom of the tank, long vertical float buoyancy force, long vertical negative buoyancy force, sight gauge with multiple optical level discrimination, multiple thermistors, tank sonic resonance spectra correlations, and the like. These methods typically have cost and complexity as their major detriment.
Liquid level is generally sensed by one of two methods: obtaining a discrete indication if a predetermined level has been reached (i.e. point sensing) or obtaining an analog representation of the level as it changes (i.e. continuous sensing). One commonly employed apparatus for continuously sensing the level of liquid, such as a fuel, in fuel storage tanks is the use of a float mechanism.
Relatively low technology methods currently being used to sense liquid levels by measurement of various properties use techniques such as: ohmic conduction between electrodes, variable force sensitive resistive conductors, float moving a plunger, and float on a mechanical pivot arm moving a wiper contact element across a wire wound or thick film resistor. The ohmic conduction method is prone to detrimental accuracy impairing contamination and electrolytic effects on the electrodes. Variable force sensitive resistors inherently suffer from poor accuracy and are seldom used. The float moving a plunger method is capable of good accuracy and precision but is cumbersome and requires some additional sensing method to convert position into a sensor signal. The method by which a float rotating a mechanical pivot arm to move an electrical wiper contact across a variable resistor type of liquid level sensor is of primary commercial interest because of its large number of existing and potential applications, relative simplicity, and low cost.
The following listed features pertaining to liquid level sensing via floats on pivot arm mechanisms, however, are less than desirable:
Requires a relatively large volume of space for pivot arm motion; PA1 Often requires specific orientation to allow for pivot arm motion; PA1 Cumbersome to install and remove; PA1 Large size; PA1 High weight; PA1 High parts count; PA1 Mechanism if often flimsy--heavier mechanism requires a larger float; PA1 Mechanism has inherent modes of mechanical vibration--potential failures; PA1 Mechanism is inherently poor in mechanical impact testing--potential failures; PA1 Relatively large pivot arm movement is reduced to smaller motion of the wiper arm on the wire wound or film resistor--loss of resolution and accuracy; PA1 Mechanical play, articulation, and tolerance of numerous components introduces hysteresis and adversely affects accuracy; PA1 Wear of articulating mechanical components increases modes of failure; PA1 Nonmetallic substrate of the wire wound or thick film resistor acts as a thermal insulator allowing heat build up in events of an incorrect electrical connection--a potential source of ignition for a dangerous incendiary incident; PA1 Small size of the wire wound or thick film resistor promotes concentration of heat generation and higher temperatures in events of an incorrect electrical connection--an additional potential source of ignition for a dangerous incendiary event; PA1 Location of the articulating electrical wiper contact with the wire wound or thick film resistor can be above liquid level such that mechanical play can potentially produce a source of sparking and/or heat production--a safety concern; PA1 Nonmetallic substrate of the wire wound or thick film resistor offers no shielding from spurious magnetic fields that can cause signal noise; PA1 Nonmetallic substrate of the wire wound or thick film resistor offers no shielding from spurious electric fields that can cause signal noise. PA1 Sturdy design; PA1 Fewer parts; PA1 Less contamination of components by integral and/or surface additives; PA1 Fewer moving parts; PA1 Low mass; PA1 Low cost; PA1 Simple assembly; PA1 Design flexibility for float assembly shape and size; PA1 Design flexibility for mounting configuration; PA1 Design flexibility for mounting location; PA1 Simple insertion; PA1 Simple removal; PA1 Requires less operation volume within the tank; PA1 Improved resolution; PA1 Decreased hysteresis; PA1 Improved accuracy and stability--due to metallization strips; PA1 Integral temperature compensation capability; PA1 Steel substrate acts as a heat sink thus reducing ignition potential; PA1 Laser trimming for minimum resistance; PA1 Laser trimming for maximum resistance; PA1 Laser trimming for transfer function profile accuracy; PA1 Steel substrate acts as a ground plane for the film resistor for reduced noise attributable to electric and/or magnetic fields; and PA1 Steel substrate performs also as a structural member.
The above detrimental points and limitations present numerous opportunities for improvements.
U.S. Pat. No. 4,779,460 discloses a sensor and system for measuring the level of a liquid and which includes a linear potentiometer having a resistance circuit element and a mechanism for changing the resistance of the circuit element including a conductor formed from a semiconducting composition having a resistance inversely proportional to the horizontal force it experiences.